Expanding on your idea and employing File::Spec to provide more friendly cross-platform execution of the subroutine ...

sub create_dirs {
    use File::Spec;
    my $file = shift;
    my $dir;
    foreach ( File::Spec->splitdir( (File::Spec->splitpath( $file ))[1
+] ) ) {
        $dir = File::Spec->catdir($dir, $_);
        next if -d $dir;
        mkdir( $dir ) || die $!;
    }
}
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perl -e 's&&rob@cowsnet.com.au&&&split/[@.]/&&s&.com.&_&&&print'