concat_latex.pl

#!/usr/bin/env perl
use strict;
use File::Slurp qw/read_file/;
use feature qw/say/;

my $main_file = shift @ARGV;
my @paths = @ARGV;

my @main_source = read_file $main_file;
for(my $i = 0; $i < $#main_source; ++$i){
  my $line = $main_source[$i];

  if($line =~ m/\\(?:include|input)\{(.+?)\}/){
    my $input_filename = $1;
    unless($input_filename =~ m/\.tex\s*$/i){
      $input_filename .= '.tex';
    }
    my @input_source = read_file $input_filename;

    splice @main_source, $i, 1, @input_source;
    $i += $#input_source;
  }
}

say @main_source;