You can declare the variable before the condition and only populate it depending on it:

my @regexes;
if ($param eq 'mod') {
    @regexes = ...
} else {
    @regexes = ...
}
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To match the entities involved in ;\s&, you need to extend the regex:

&[^;]+;\s&[^;]+;
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Here's a complete example. Running it with script.pl 1.xml searches for the original regexes, specifyin any true parameter (e.g. script.pl 1.xml 1) after the file uses the modified regexes. I also added the grouping parentheses to the matching and $1 to the output to show which part of the string actually matched the regex.

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

my ($infile, $param) = @ARGV;
my @regexes = $param ? (qr/&[^;]+;\s&[^;]+;/)
                     : (qr/–§/,
                        qr/–Ü/,
                        qr/ߧ/);

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

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

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

print {$out} "Find pair of entities without/with separating space\n\ni
+nput file: ";
print {$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;
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First of all, many thanks for your valuable input! I learn a lot from your comments :-)

I understood that my ($infile, $param) = @ARGV; is a smart way to read two input parameters from the command line. What happens if the second parameter is missing, i.e. perl entityPairs.pl input.xml? Will that mean that the condition in ternary operator expression evaluates to false and the values after ":" are taken?

I've got two issues:

• The output only shows $regex, but not $1 (the current value), so for example &[^;]+;\s&[^;]+;: for each occurrence in the original file and for example –§: in the modified file.

• There is a warning with respect to the line that prints the occurrences.

  print {$out} "$regex: $1\n" while $xml =~ /$regex/g;
  

  Use of uninitialized value $1 in concatenation (.) or string at entityPairs.pl line 42, <$in> line 1 (#1)
  (W uninitialized) An undefined value was used as if it were already defined. It was interpreted as a "" or a 0, but maybe it was a mistake. To suppress this warning assign a defined value to your variables.

  To help you figure out what was undefined, perl will try to tell you the name of the variable (if any) that was undefined. In some cases it cannot do this, so it also tells you what operation you used the undefined value in. Note, however, that perl optimizes your program and the operation displayed in the warning may not necessarily appear literally in your program. For example, "that $foo" is usually optimized into "that " . $foo, and the warning will refer to the concatenation (.) operator, even though there is no . in your program.

FYI, the current version of the script:

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

print "Find pair of entities without/with separating space\n";

# read input file and param ('mod' = for modified files)
my ($infile, $param) = @ARGV;

my @regexes = $param ? (qr/–§/,
                        qr/–Ü/,
                        qr/ߧ/)
                     : (qr/&[^;]+;\s&[^;]+;/);

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

#read input file in variable $xml
my $xml;
{
  local $/ = undef;
  $xml = <$in>;
}

#define output file
open my $out, '>', 'pairs.txt' or die $!;

#output statistics
print {$out} "Find pair of entities without/with separating space\n\ni
+nput file: ";
print {$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;
[download]

I've got two issues:

Actually, you have only one issue with 2 symptoms. Both symptoms are because you are not capturing the results of the match. Use brackets to do that. eg:

$ perl -we '$regex = qr/foo/; print "$regex: $1\n" while "foob" =~ /$r
+egex/g;'
Use of uninitialized value $1 in concatenation (.) or string at -e lin
+e 1.
(?^:foo): 
$ perl -we '$regex = qr/(foo)/; print "$regex: $1\n" while "foob" =~ /
+$regex/g;'
(?^:(foo)): foo
$
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🦛