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 $string =~ /^ *$/;
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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 $string =~ /^\s*$/;
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\s matches whitespace, so this does the same as the last regex, but matches any whitespace (not just space characters).