If you only want to count the lines then use something like

my @myfile = <File>;
my $lines = @myfile;
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but that was just guessing what you are doin.
But to go on with your idea you need to

undef $/;           # enable "slurp" mode
$_ = <FH>;          # whole file now here
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which is an excerpt from perlman:perlvar

Have a nice day
All decision is left to your taste