You mean just the space character? Not for any whitespace?
To check if a string contains only space characters, use
my $string = " ";
$string = "Default" if $string =~ /^ *$/;
This sets $string to "Default" if $string consists of
0 or more space characters, and only space characters;
if $string is " a " or something such, the regular
expression won't match.
If you're looking to match strings that consist only of
whitespace (no non-whitespace characters), use:
my $string = " \t \n ";
$string = "Default" if $string =~ /^\s*$/;
\s matches whitespace, so this does the same as the last
regex, but matches any whitespace (not just space characters). | 