"How do I make a single step?"

"You start walking and then stop after the first step."

foreach $key (sort(keys %in)) {
    print MAIL "\t", $key, " = \t", $in{$key}, "\n";
    last;
}
close MAIL;
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Or, as an approach that is a bit more verbose yet without the loop:

my @keys = sort keys %in;
print MAIL "\t", $key[0], " = \t", $in{$key[0]}, "\n";
[download]

Or, as an approach that is a bit more transparent and doesn't use the array:

(my $first) = sort keys %in;
print MAIL "\t", $key, " = \t", $in{$key}, "\n";
[download]