$mystring = pack('H*', '0D0A...');

##</code><code>##

# Ignores whitespace
($mystring = '0D 0A ...') =~ s/([0-9a-fA-F]{2})/chr($1)/g;

##</code><code>##

# Multiline and ignores whitespace
($mystring = <<'__EOI__') =~ s/([0-9a-fA-F]{2})/chr($1)/g;
0D 0A ...
0D 0A ...
0D 0A ...
...
__EOI__