only first pattern found is displayed now. That's not a problem, but I'd also like to know how to display all patterns if a file contains more than one.

That's easy -- instead of using an "if" statement like this:

if (/($regex_string)/is) {
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while (/$regex_string)/isg) {
    print "\nmatched in $arg1:\n==$1==\n";
}
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$1 will show all wildcarded text between part1 and part2 and not only the pattern part1.*part2 as it should.

What makes you think it "should" display the string "part1.*part2"? When using the capture variables ($1, $2, ...), the normal situation is to want the actual (complete, literal) string that matched the regex, rather than the regex string with its wildcards.

If you want the wildcard-enabled regexes in your list to return a specific constant string, you'll probably want to include that string in your regex list file, store those replacement strings with their regexes in a separate hash, and add some logic in the while loop shown above that will replace any given matching string with the appropriate constant replacement string. Here's an adapted version the three files involved:

part1.*?part2    part1.*?part2
Foo bar
Other pattern
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hghghgghghh
part1
fff
part2
blah blah
Foo bar
more noise
Other pattern
another match for
part1
stuff I
don't want
to see part2
blabla
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#!/usr/bin/perl -w

use strict;

my $patterns = "patterns.txt";
my $arg1 = shift;
my $regex_string = '';
my %patterns;

open (PAT, '<', $patterns) or die "$patterns: $!\n";
while (<PAT>) {
    chomp;
    my ( $pattern, $replace ) = split '\t';
    $patterns{$pattern} = $replace if ( $replace );
    $regex_string .= $pattern . '|';
}
close(PAT);

chop $regex_string;  # remove string-final '|';

open( FILE, "<", "$arg1") or die "$arg1: $!\n";
$_ = do { local $/; <FILE> };
close(FILE);

while ( /($regex_string)/gis ) {
    my $match = $1;
    for my $replc ( keys %patterns ) {
        last if ( $match =~ s/$replc/$patterns{$replc}/is );
    }
    print "\nmatched in $arg1:\n==$match==\n";
}
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