#! /usr/bin/perl
use strict;
use warnings;

my @output;
my $count = 0;

# read input data line by line
while ( my $line = <DATA> ) {

    # one block in input data file starts with "Time Step X" and ends with empty line
    if ( $line =~ m{^Time Step \d+$} .. $line =~ m{^\s*$} ) {

        # save data blocks 
        push @output, $line;

        # process data block when end of block
        if ( $line =~ m{^\s*$} ) {

            for my $i ( 1 .. 24 ) {

                # replace number in time step line
                $output[0] =~ s/\d+$/ ++$count/e;

                # print data block to STDOUT
                print @output;

            }

            # reset @output for next data block
            @output = ();
        }
    }
}

__DATA__
Time Step 1
a
b
c
d

Time Step 2
e
f
g
h

Time Step 3
i
j
k
l