Show us a code sample that we can run and that doesn't need anything outside the sample script. You may find Re^3: loop through file checking for session variable provides a useful starting point.

Use strictures (use strict; use warnings;).

if($Session->{'usrSystem'})=~/$line/g;
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is not syntacticly correct. If it is intended as a statement modifier it needs a statement before the if. If it is an if it needs a block and the expression must be in parenthesis.

Use the three parameter open:

my $site_file = 'c:\sitefile.txt';

open my $Data, '<', $site_file or die "Horrible death";
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