You probably want something like this (UNTESTED)

#!/usr/local/bin/perl
use warnings;
use strict;
use File::Find;
use File::Copy;

@ARGV == 2 or die "Usage: $0 sourcedirectory targetdirectory\n";
my ( $sourcedirectory, $targetdirectory ) = @ARGV;

find sub {
    return unless -f && /\.txt\z/i;
    copy( $_, "$targetdirectory/$_" );
    }, $sourcedirectory;
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