I haven't golfed in a while, so here's Masem's code in one line:

(my $secs = "5H3M17S") =~ s/((\d+)H)?((\d+)M)?((\d+)S)?/3600*$1 + 60*$
+3 + $5/e;
[download]

And just to be different:

use strict;

my $time     = "5H3M17S";
my $secs     = 0;
my $exponent = 0;

map { $secs += $_ * 60 ** $exponent++ } reverse(split(/[HMS]/, $time))
+;
[download]

-Matt