Something like (untested):

use 5.010;
use autodie;
my $CHUNK_SIZE = 1024 * 1024;   # Read 1Mb chunks.
open my $fh, "<", $filename;
open my $out, ">", $outfilename;
my $buffer;
while (read $fh, my $chunk, $CHUNK_SIZE) {
    $buffer .= $chunk;
    while ($buffer =~ m{-/([a-zA-Z0-9]{5,20})\?srt=/}g) {
        say $out $1;
        substr($buffer, 0, pos($buffer)) = "";
    }
    substr($buffer, 0, -26) = "";  # Keep last characters
}
__END__
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