This should work for any number of files, up to the maximum you can have open.

use List::Util qw( first );

my @files = ( 'FILE1', 'FILE2' );

my @handles;
foreach my $file ( @files ) {
    open my $fh, '<', $file or die "Can't read $file: $!";
    push @handles, $fh;
}

my $first_handle = shift @handles;

while ( my $line = <$first_handle> ) {
    if ( defined first { $line ne <$_> } @handles ) {
        die "There's a difference.";
    }
}
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