Perl does have a very fine regex, as demonstrated below by BrentDax, which is what I was thinking of doing initially, forgetting, at the time, that feature of split().

No matter how you slice it, though, regex or otherwise, you should put the code into a sub-routine. At least, anyone who is concerned about maintainability would make sure to do that. Having a compact regex is not a license to cut and paste it all over your code, wherever it is required.

If you're afraid of this code looking like Java, well, that isn't too hard to fix:

my %time_value  = ( d => 86400, h => 3600, m => 60, s => 1 );

sub string_to_time
{
        my ($time) = @_;
        my $value  = 0;

        map { $value += $1*$time_value{$_} if ($time =~ s/(\d+)$_//) }
+ sort keys %time_value;

        return ($value && !length $time)? $value : undef;
}

sub time_to_string
{
        my ($time) = @_;
        my $value  = '';

        (defined $time && $time > 0) || return undef;

        map { $value .= int($time/$time_value{$_}).$_ and $time %= $ti
+me_value{$_} if ($time > $time_value{$_})} sort keys %time_value;

        return $value;
}
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So much for readability and maintainability though.