while (my $line = <INFILE>) {
    chomp $line;
    if ($line =~ /^$ARGV[1]/){
        $filter_count++;
        push(@rawfile,$line);
    }
}
[download]

A little bit more fine-tuning:

• If the line does not match, there is no point in chomping it. So move chomp into the if block. (This assumes the search pattern has no problems with the trailing line-end character(s) in $line.)
• Using a variable inside the regexp forces perl to recompile it several times (I think). my $pattern=qr/^$ARGV[1]/; in front of the while loop and using $line=~$pattern in if would avoid that.
• If the search is for a keyword at the first character of the line, and not a pattern, using a regexp may be slower than a simple string operation. Try using if (index($line,$ARGV[1])==0) or if (substr($line,0,length($ARGV[1])) eq $ARGV[1]) instead.

Hint: Use Devel::NYTProf to locate hot spots in your code.

Alexander