In addition to the Regex suggestion - I'd suggest avoiding the @lines altogether.

Instead of "push @lines, $line (Storing it), why don't you call convert_to_ttf($line) at that point - this way, you can avoid the store .. and .. process .

Similarly, instead of storing @converted lines, then printing them, do that at the same place - so your code looks like:

open my $fh ... etc
while (<>) {
    $line .= $_;
    if (is_complete_line($line)) {
         print $fh convert_to_ttf ($line);
        $line = '';
    }
    
}
close $fh;
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