How about this?

#! perl
use strict;
use warnings;

my $s = 'XYX_WT_7_25h_Trimmed_75.SAM';

print $1 if $s =~ / ( _ [^_]+ _ [^_]+ ) $ /x;
[download]

Output:

23:31 >perl 1687_SoPW.pl
_Trimmed_75.SAM
23:32 >
[download]

Hope that helps,