PPI can do most of the work for you. The code below illustrates how to use PPI to extract the find and replace patterns.

Note: The current PPI, version 1.103, has a bug that I have worked around in my code. I am submitting a patch to fix the bug; when a version is released without the bug, I will post an update with the new version number.

Working, tested code:

use strict;
use warnings;
use PPI;

my $source_code = <<'END_OF_SOURCE_CODE';
    $text =~ s#Find \#Text#Replace Text#;
    $text =~ s/Find \/Text/Replace Text/;
    $text =~ s{Find \}Text}{Replace Text};
    $text =~ s{Find \}Text}  {Replace Text};
    $text =~ s[(Find) (\}Text)]  <Replace Text>io;
END_OF_SOURCE_CODE

my $Document = PPI::Document->new(\$source_code)
  or die;

my $subs = $Document->find('PPI::Token::Regexp::Substitute');

#use Data::Dumper; $Data::Dumper::Useqq = 1;
#print Dumper $subs;
#exit;

foreach ( @{ $subs } ) {
    my $type = $_->{'braced'} ? 'Braced' : 'Not Braced';
    my ( $find_text, $repl_text ) = extract_subst_strings($_);
    print "\n$type\n";
    print "find_text = '$find_text'\n";
    print "repl_text = '$repl_text'\n";
}

# Fails on braced substitute statements in PPI 1.103
#sub extract_subst_strings {
#    my %s = %{ $_[0] };
#
#    return map {
#        substr( $s{'content'}, $_->{'position'}, $_->{'size'} )
#    } @{ $s{'sections'} };
#}
sub extract_subst_strings {
    my ($substitute_statement) = @_;
    my %s = %$substitute_statement;

    warn unless    $s{'_sections'}   == 2;
    warn unless @{ $s{ 'sections'} } == 2;

    my $content  =    $s{'content' };
    my @sections = @{ $s{'sections'} };

    my ( $find_text, $repl_text );
    if ( $s{'braced'} ) {
        $find_text = substr(
            $content,
            $sections[0]{'position'},
            $sections[0]{'size'},
        );
        $repl_text = substr(
            $content,
            $sections[1]{'position'} - $sections[1]{'size'} - 1,
            $sections[1]{'size'},
        );

    }
    else {
        ( $find_text, $repl_text ) = map {
            substr( $content, $_->{'position'}, $_->{'size'} )
        } @sections;
    }

    return ( $find_text, $repl_text );
}
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