Assuming you want output only if there is a matching chapter and figure number:

use strict;
use warnings;
# avoid using $a and $b
my $f='figure1 vdfsvh figure3 vdsvjjh'; 
my $ch='chapter 3 svbjjvfh chapter5';

my @figs  = $f  =~ /figure\s*(\d+)/g;
my %chaps;
$chaps{$_}++ for ( $ch =~ /chapter\s*(\d+)/g ); 

for my $i (@figs) {
        print "href=\"chapter $i.html#figure $i\">figure $i\n" if exis
+ts $chaps{$i};
}
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