See Path::Tiny, File::Find::Rule

#!/usr/bin/perl --
use strict; use warnings;
use Path::Tiny qw/ path /;
use File::Find::Rule qw/ find /;
Main( @ARGV );
exit( 0 );
sub Main {
#~     MainReaddir( @_ );
    MainRule( @_ );
}
sub MainRule {
    my( $dir , $mergeinto ) = @_;
    my $mergefh = path( $mergeinto )->opena_utf8;
    my $readdir = path( $dir )->realpath;
    my @entries = find( 
        file     =>
        maxdepth => 1,
        name     => qr/\.txt$/,
        in       => $readdir,
    );
    for my $name ( @entries ){
        my $entry = path( $name );
        if( $entry->is_file and $name =~ /\.txt$/ ){
            my $inputfh = $entry->openr_utf8;
            Mergerations( $mergefh, $inputfh );
            close $inputfh;
        }
    }
    close $mergefh;
}
sub MainReaddir {
    my( $dir , $mergeinto ) = @_;
    my $mergefh = path( $mergeinto )->opena_utf8;
    my $readdir = path( $dir )->realpath
                        ->iterator( { qw/ recurse 0 / });
    while( my $entry = $readdir->() ){
        if( $entry->is_file and $entry =~ /\.txt$/ ){
            my $inputfh = path( $entry )->openr_utf8;
            Mergerations( $mergefh, $inputfh );
            close $inputfh;
        }
        
    }
}
sub Staterator {
    my( $out, $in ) = @_;
    ...
}
__END__
[download]

see perlintro, also chromatics free book Modern Perl a loose description of how experienced and effective Perl 5 programmers work....You can learn this too.
Learn Perl in about 2 hours 30 minutes