#!/bin/perl -w
use strict;
foreach( <DATA>)
  { s{^\d+_                          # start then number(s) then _
       ((?:\D*(?:\d(?!\d*_))?)*)     # non_digits then optionnaly
                                     #   a digit _not_ followed by digits and _
                                     # repeat, rince...
       \d+_                          # the second digit _ sequence
       (.*)}                         # the rest of the string
     {$1$2}x;
    print;
  }

__DATA__
1_abc/2_deg/bla_30_31_blah
1_ab1c/2_deg/bla_30_31_blah
1_ab1c/22_deg/bla_30_31_blah
1_a2b33c/22_deg/bla_30_31_blah
a_ab1c/b_deg/bla_30_31_blah