Ouch. You don't have to read all lines in at once first. This line is doing that since the angle bracket operator is invoked in array context:

my @file = <FH>;
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To read a line at a time do this instead:

local *FH;
open(FH, "/path/to/file") || die "Can't open file: $!";
while (<FH>)
{
    # line is in $_
    if ($_ eq $external_info)
    {
        print "great\!";
        last;
    }
}

close(FH);
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