#!/usr/bin/perl -lw
use strict;
# [id://735593]

my $string = '<img id="image-1" src="http://site.com/a.gif" />';
my $regex = qr|<img id="image-(\d)" src="http://site.com/(\w\.\w+)" />|;    # works
#                                                         ^^^^^^^           < modified here
if ( $string =~ /$regex/ ) {
  print "\$regex matches: $1 and $2";
} else {
  print "No match in $regex";
}

my $OPregex= qr|<img id="image-(\d*) src="http://site.com/(\w*)" />|;     # OP's with (added) terminating "|;"
if ( $string =~ /$OPregex/ ) {
  print "\$OPRegex matches: $1 and $2";
} else {
  print "no match in \$OPRegex";
}