I would use a substitution operator to get rid of the leading "gpu" like so:

$first_line =~ s/^gpu//;
print FOUND $first_line;
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See perlop for more about substitution and perlre for more on regular expressions. Also, it doesn't look like you are using strict. Strict forces you to used good coding practices and helps you catch a lot of coding errors. Always, always put "use strict" shortly after the shebang line.

Update: If you just want to get rid of the first 3 arbitrary characters, use s/^.{3}//;