use strict;
use warnings;
use Data::Dumper::Simple;

my $stop = qr/[\s:]/;  # modify as appropriate!
my @lines;
my $pattern;

## test one..
@lines = split /\n/, <<'EOD';
Error 123 on SystemA file not found error
Error 123:on SystemB file not found error
Error 123 on SystemC file not found error
EOD

$pattern = get_pattern( \@lines );
print Dumper($pattern);

## test two..
@lines = split /\n/, <<'EOD';
Error 124 on User1:FileA no space left
Error 124 on User2:FileB no space left
Error 124 on User3:FileC no space left
EOD

$pattern = get_pattern( \@lines );
print Dumper($pattern);


sub get_pattern {
    my @lines = @{ +shift };    # this should take a copy..

    my @known_words = split /($stop)/, shift @lines;
    my @pattern     = @known_words;  # assume everything matches..
    my %modified    = ();            # track indices
    my $count = 0;     # $x incrementor


    LINE:
    foreach my $line (@lines) {
        my @words = split /($stop)/, $line;

        unless ( @words == @known_words ) {
            warn "ignoring (word count does not match first line): $line";
            next LINE;
        }

        WORD:
        foreach my $i ( 0 .. $#words ) {
            next WORD if $modified{$i};     # already noted this spot

            my $this = $words[$i];
            next WORD if $this =~ $stop;    # questionable.. are all stops 'equal'?

            my $that = $known_words[$i];
            next WORD if $this eq $that;    # everything looks ok so far

            $pattern[$i] = '$' . ++$count;
            $modified{$i}++;
        }
    }

    return join '', @pattern;
}