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sub paste {
    my @fh = @_;
...
        return join ' ', @line;
    }
}

open my $keys, "keys.txt" or die "couldn't open keys.txt";
open my $vals, "vals.txt" or die "couldn't open vals.txt";
my $combined = paste($keys,$vals);
print $combined->(), "\n" for 0..4;

Joe 5
Bob 3
Sally 7
Bob 1
Fred 1

sub NEXTLINE { $_[0]->() }
print NEXTLINE($combined), "\n" for 0..4;