use strict;
use warnings;
my $infile = "filename.txt";
my $outfile = "filename.out";

open my $IN, "<", $infile or die "Could not open file $infile : $!";
open my $OUT, ">", $outfile or die "Could not open file $outfile : $!";
my $step_num = 0;

while (<$IN>) {
    if (/-----------step [\d]\./){
        $step_num ++;
        s/-----------step \d\./-----------step $_\.$step_num/;
    }
    print $_ $OUT;
}
close $IN;
close $OUT;