use strict;
use warnings;

my $file1 = <<'END_FILE';
1 G
2 F
4 H
8 T
END_FILE
my $file2 = <<'END_FILE';
5 Y
6 R
7 E
8 I
END_FILE
my $file3 = <<'END_FILE';
2 H
4 T
3 G
8 R
END_FILE

my @fileHandles;

open $fileHandles[@fileHandles], '<', $_ for \$file1, \$file2, \$file3;
printLines (@fileHandles);
close $_ for @fileHandles;


sub printLines {
    my @fileHandles = @_;
    my $moreLines = 1;

    while ($moreLines) {
        my $line = '';

        for my $fh (@fileHandles) {
            $line .= '  ' if length $line;
            $line .= <$fh> or return;
            chomp $line;
        }

        print "$line\n";
        $line = '';
    }
}