Presumably, you have some definition of the number of lines that corresponds to a page. If you are on a unix box, you can get the number of line with wc:

my @lines = `wc -l $data_file`; 
@lines[0] =~ /(\d+)\s/;
my $linecount = $1;
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I know less about windows boxes, but you can still always open the file and read through it, adding up the number of lines as they go by:

my $linecount=0;
open FILE, $data_file;
while (<FILE>) {
    $linecount++;
}
close FILE;
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Easy enough, though I suspect there may be a better way.

Scott

Updated wc to get only the number.

update2: It's funny how it's always the simple questions: one thing that occurred to me is that this assumes there are no lines that will wrap on printing. If that is a good assumtion, fine. If not, code will have to be added to check the length of the line. If it is long enough to wrap, $linecount should be incremented by 2.