Perl-only solution:

#!/usr/bin/perl -w
use strict;

${^WIN32_SLOPPY_STAT}++;    # Speed up the 'stat' call on Windows

if ((stat "/tmp/A")[9] > (stat "/tmp/B")[9]){   # Compare mtime stamps
    print "A is newer than B; discarding B.\n";
    unlink "B" or die "B: $!\n";
}
else {
    print "B is newer than A; replacing A.\n";
    unlink "A" or die "A: $!\n";
    rename "B", "A" or die "Copying B->A: $!\n";
}
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