Ok ok ok,
then take this:

use strict;
use warnings;
my $str = '6AG3_608_FGG_20100124_20120923418_20126254447_201262512026.
+txt';
$str =~ m/(.*?)(_\d{11,}?.*)(\.\w+)/;
print $1, "\n";
print $2, "\n";
print $3, "\n";
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