The Pilgrims trailed 2-0 at half-time courtesy of goals from Robert Paratore and David Hibbert but the momentum changed when home debutant Jake Hibbs was handed his marching orders.
Piergianni pulled a goal back from the resulting Dylan McEvoy free-kick and Southwell equalised 11 minutes later to ensure a share of the spoils.
The result extended United's unbeaten run to four and took their points tally in October to 10 from a possible 12.
Piergianni had returned at the heart of the United defence while Cameron Johnson was handed his first league start after an impressive cameo against Alfreton Town seven days previously.
Telford were straight on the offensive and Boston required a tremendous save from Henrich Ravas to deny Paratore before Piergianni slammed the loose ball away in the nick of time.
Connor McCarthy's snapshot then flew narrowly wide before the hosts deservedly took the lead in the seventh minute with a terrific curling finish from PARATORE from just outside the box giving Ravas no chance.
Boston responded with home keeper James Montgomery denying Scott Garner following Southwell's low cross while Piergianni's header was inches away from restoring parity, rattling the bar, following Grant Roberts' corner.
It could have been 1-1 but the Pilgrims were 2-0 down seconds later with HIBBERT sailing home a beauty from 25 yards.
The visitors went close again before the break with Mark Jones' excellent header flying inches wide following McEvoy's deep free-kick.
Hibbs was booked in first half stoppage time for leaving stud marks on McEvoy's lower leg and was subsequently dismissed eight minutes after half-time for halting Liam Marrs' surge forward.
Hibbs took his time to leave the field and when McEvoy eventually delivered the wide free-kick, PIERGIANNI got up well to power home his fourth of the season.
The Pilgrims equalised on 66 minutes when SOUTHWELL superbly guided home Jordan Richards' lovely cutback from the left.
The visitors sensed victory, but once sub Nicky Walker had been denied by a terrific Curtis Tilt block and Southwell sent the follow-up over the bar, the best chance of the remainder of the game fell to Telford sub Ryan Rowe, who saw his curling effort brilliantly clawed behind by Ravas.
Telford (4-4-2): James Montgomery; Dwayne Samuels, Graham Hutchison (Ryan Rowe 68), Paul McCone, Darren Campion; Jake Hibbs, Jack Rea, Robert Paratore (Josh Ashby 76), Sean Clancy; David Hibbert (Curtis Tilt 63), Connor McCarthy. Subs (not used): Kyle Brownhill, Niall Flint.
Boston (4-2-1-3): Henrich Ravas; Liam Marrs, Scott Garner, Carl Piergianni, Dylan McEvoy; Jordan Richards, Grant Roberts; Cameron Johnson (Nicky Walker 56); Mark Jones, Dayle Southwell, Ryan Robbins (Kyle Dixon 77). Subs (not used): Lartey Sarpong, Jason Alexander (gk).
Referee: Joe Hull.
Attendance: 1,158 (65 visitors).