Tie::File is of course the way to do it. The old, hardcore, quickhack, low memory, no extra module, the-way-they-used-to-do-it-in-the-80's way is:

#!/usr/bin/perl

open INFILE, "<infile";
open OUTFILE, ">tmpfile";    # Create your temporary names with care, 
+or die a painful race-condition death.

print OUTFILE qq!Here you print the new stuff.
...
!;

while (<INFILE>) {
 print OUTFILE $_;
}

close INFILE;
close OUTFILE;
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and then rename the tempfile.