Use pack with the Z template.

Update: An example.

johngg@aleatico:~$ perl -Mstrict -Mwarnings -E '
my $str    = q{example.me};
my $length = length $str;
my $padTo  = 4;
my $rem    = $length % $padTo;
my $padLen = $length + ( $rem ? $padTo - $rem : 0 );
print pack qq{Z$padLen}, $str;' | hexdump -C
00000000  65 78 61 6d 70 6c 65 2e  6d 65 00 00              |example.m
+e..|
0000000c
[download]

Update 2: Don't use this, if the string is already aligned to a 32-bit boundary the Z template will overwrite the last character with a NULL is it seems to always produce a C-style NULL-terminated string.

Update 3: Removed the strike-through! The solution is to only pad with pack when necessary but it does begin to look messy.

johngg@aleatico:~$ perl -Mstrict -Mwarnings -E '
my $str    = q{myexample.me};
my $length = length $str;
my $padTo  = 4;
my $rem    = $length % $padTo;
print $rem
   ? pack( qq{Z@{ [ $length + $padTo - $rem ] }}, $str )
   : $str;' | hexdump -C
00000000  6d 79 65 78 61 6d 70 6c  65 2e 6d 65              |myexample
+.me|
0000000c
[download]