You could use File::Basename, split and tr:

use File::Basename;
$record = "FRAG 1 1 2000000 0 0 0 0 \\\\172.20.13.49\\backup\\rbgmst02
+_dd2\\stu1\\CPOSTA_1221170039_C1_F1 rbgmst02 65536 0 0 -1 0 *NULL* 12
+22379639 1 65537 0 0 0 0 0 0 0";
if($record =~ m|FRAG|)
{
    my $str = (split " ", $record)[8];
    $str =~ tr[\\][/];
    print basename($str),"\n";
}
__END__
CPOSTA_1221170039_C1_F1
[download]

Alternative:

$record = "FRAG 1 1 2000000 0 0 0 0 \\\\172.20.13.49\\backup\\rbgmst02
+_dd2\\stu1\\CPOSTA_1221170039_C1_F1 rbgmst02 65536 0 0 -1 0 *NULL* 12
+22379639 1 65537 0 0 0 0 0 0 0";
if($record =~ m|FRAG|)
{
    $record =~ /(?<=\\)(\w+)(?:\s|$)/;
    print "$1\n";
}
__END__
CPOSTA_1221170039_C1_F1
[download]

See perlre.