Thank you both for the response! To elaborate on my question, let's assume that the input file contains a binary number of:

"0101_1111_1111_0000_0111_0000_1111_1010"

Output should be:

"1_0101_1111_1111_0000_0111_0000_1111_1010"

This is purely binary input. I only added the "_" for readability. I understand that we cannot write a file on non byte boundaries. So if I would always have 7 extra bits when this is written to the file. This would mean that my output file will look like:

"0000_0001_0101_1111_1111_0000_0111_0000_1111_1010"

When I run the code below, I get an output of:

"0000_0001_0000_0000_0000_0000_0000_0000_0000_0000"

I think I am still misusing the pack command. What am I missing? Code below:

#!/usr/bin/perl 
my $inputfile = $ARGV[0];
my $outputfile = $ARGV[1];
my $buff = "";
my $mybuff = "";

open (PARSER_FILE, $inputfile) 
    or die "Couldn't open $inputfile for reading";
binmode(PARSER_FILE);
open (OUT_FILE, ">", $outputfile) 
    or die "Couldn't open $outputfile for writing";
binmode(OUT_FILE);
read(PARSER_FILE, $buff, 4);

$outbuff = unpack("b32", $buff);
syswrite OUT_FILE, pack("b33", 1, $outbuff);
close(OUT_FILE);
close(PARSER_FILE)
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