In addition to the problem of the extraneous underscore in  /rev_(\d)+/ in the OP, note also that  (\d)+ will not match what I think you think it will if there is more than one digit present:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'foorev_123bar';
 ;;
 print qq{captured '$1'} if $s =~ m{ rev_ (\d)+ }xms;
"
captured '3'
[download]

Capture multiple digits with:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'foorev_123bar';
 ;;
 print qq{captured '$1'} if $s =~ m{ rev_ (\d+) }xms;
"
captured '123'
[download]

One previous reply has implicitly noted this problem. (Update: haukex has also addressed this below.)