perl -Mre=debug -e '$_ = "foobar#2"; s/#[0-9]+$//'
Compiling REx "#[0-9]+$"
Final program:
   1: EXACT <#> (3)
   3: PLUS (15)
   4:   ANYOF[0-9][] (0)
  15: EOL (16)
  16: END (0)
anchored "#" at 0 floating ""$ at 2..2147483647 (checking anchored) minlen 2 
Guessing start of match in sv for REx "#[0-9]+$" against "foobar#2"
Found anchored substr "#" at offset 6...
Found floating substr ""$ at offset 8...
Starting position does not contradict /^/m...
Guessed: match at offset 6
Matching REx "#[0-9]+$" against "#2"
   6 <foobar> <#2>           |  1:EXACT <#>(3)
   7 <foobar#> <2>           |  3:PLUS(15)
                                  ANYOF[0-9][] can match 1 times out of 2147483647...
   8 <foobar#2> <>           | 15:  EOL(16)
   8 <foobar#2> <>           | 16:  END(0)
Match successful!
Freeing REx: "#[0-9]+$"