use strict;
use warnings;

my %words;

for my $fnum ( 1 .. 4 ) {
    open( I, '<', "File$fnum.txt" ) or die "File$fnum.txt: $!\n";
    while (<I>) {
        chomp;
        s/^[\s\d.]+//;  # remove initial digit(s), period, whitespace, if any

        $words{$_} .= $fnum
            unless ( exists( $words{$_} ) and $words{$_} =~ /$fnum$/ );
    }
}

print "Words found in all four input files:\n";
for ( sort keys %words ) {
    print "$_\n" if ( $words{$_} eq '1234' );
}