use warnings;
use strict;

my %filehash;

my @files = qw(F1.dat F2.dat);

for my $filename (@files) {
    open my $filehandle, '<', $filename or die "Can't open $filename $!\n";
    while (<$filehandle>) {
        my ( $word, $remainder ) = split ' ', $_, 2;
        $filehash{$word} = $remainder;
    }
    close $filehandle;
}

open my $merged, '>', 'merge.dat' or die "Can't open merge.dat:$!\n";
for ( sort keys %filehash ) {
    print $merged "$_ $filehash{$_}";
}