my @regexes;
if ($param eq 'mod') {
    @regexes = ...
} else {
    @regexes = ...
}

##</code><code>##

&[^;]+;\s&[^;]+;

##</code><code>##

#!/usr/bin/perl
use warnings;
use strict;

my ($infile, $param) = @ARGV;
my @regexes = $param ? (qr/&[^;]+;\s&[^;]+;/)
                     : (qr/&ndash;&sect;/,
                        qr/&ndash;&Uuml;/,
                        qr/&szlig;&sect;/);

open my $in, '<', $infile or die "Cannot open $infile for reading: $!";

my $xml;
{
    local $/;
    $xml = <$in>;
}

open my $out, '>', 'pairs.txt' or die $!;

print {$out} "Find pair of entities without/with separating space\n\ninput file: ";
print {$out} "$infile";
print {$out} "\n========================================================================\n\n";

for my $i (0 .. $#regexes) {
    my $regex = $regexes[$i];
    $regex =~ s/^\(\?\^://;
    $regex =~ s/\)$//;
    print {$out} "$regex: $1\n" while $xml =~ /($regex)/g;
 }

close $in;
close $out;