This will find the literal text "GL*{3}G" in your fasta file, and will print the line number as well as the actual line where it was found. If that trigger text was supposed to be a regular expression you'll need to show us an actual regular expression that is Perl compatible.

perl -nle 'm/\QGL*{3}G\E/ && print "$.: $_"' file.fasta
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If you had something else in mind, please post the code you've started with so that we can help you with it. Also post sample input and output, and don't forget to wrap code and IO samples in code tags per Writeup Formatting Tips.