#
# Arguments: 
#    $a1 == reference to an array of scalars
#    $a2 == reference to an array of scalars
# Returns:
#    an array of scalars that are the result of 
#  prepending the contents of $a2 to the elements of $a1
#  or undef if the number of elements of $a1 and $a2 
#  are not the same.

sub prepender{
    my ($a1,$a2)=@_;
    my @ax=@$a1; # Dereference only for clarity.
    my @ay=@$a2; #    same as above

    return undef if $#ax != $#ay;

    return map { $ay[$_] . $ax[$_] } (0..$#ax);
}

#
# Sample call
my @array=('A'..'G');
my @array1=(10..($#array1+9));

my @array2=prepender(\@array,\@array1);