You can use the built-in stat function to determine the modification time of a file.

use strict;
use warnings;

my $from = "first.txt";
my $to = "new.txt";
copy_if_newer($from, $to);

sub copy_if_newer
{
    my ($from, $to) = @_;
    
    # test for file modification time
    return 0 if (stat $from)[9] <= (stat $to)[9];

    # do the actual file copy here with File::Copy, etc.

    # return 1 to indicate success
    return 1;
}
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