#!/usr/bin/perl -w
use strict;

my $var = "$GPGLL,577.015,N,01201.076,E,"; #wrong!

or

my $var = "\$GPGLL,577.015,N,01201.076,E,"; #better.  Note the escape.

or

my $var = '$GPGLL,577.015,N,01201.076,E,';