#!/usr/bin/perl

use Modern::Perl '2014';

# generate some filenames
my @inputfiles = map { "data$_.txt" } 1 .. 3;

my $inputdata = {};

# read files into $inputdata
foreach my $filenumber (0 .. $#inputfiles) {

    open my $HANDLE, "<", $inputfiles[$filenumber]
        or die "Cannot open $inputfiles[$filenumber]: $!\n";

    while(<$HANDLE>) {
        chomp;

        my ($freq, $data) = split /\s+/, $_, 2;
        $inputdata->{$freq}->[$filenumber] = $data;

    }

    close $HANDLE
        or warn "Cannot close $inputfiles[$filenumber]: $!\n";

}

# write combined output
open my $OUTPUT, ">", "combined.txt"
    or die "Cannot open combined.txt: $!\n";

foreach my $freq (sort keys %{ $inputdata }) {
    say $OUTPUT "$freq ", join " ", @{ $inputdata->{$freq} };
}

close $OUTPUT
    or warn "Cannot close combined.txt: $!\n";