Works for me if I use STDIN or STDERR instead of STDOUT. Those handles are still connected to the terminal the parent script is started from¹, while STDOUT (in the child) is connected to the pipe (for which the used TIOCGWINSZ ioctl call is invalid).

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¹ unless you've reopened them to somewhere else in the Perl code, or redirected them to/from a file in the calling shell, of course...

Update:

_____ rcmd.pl _____

#!/usr/bin/perl
use Term::Size;

my ($col, $row) = Term::Size::chars *STDIN{IO};

print "$col x $row\n";
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_____ 699419.pl _____

#!/usr/bin/perl

my $rcmd = "./rcmd.pl";

if (open my $cmdh, "$rcmd |") {
    print <$cmdh>;
}
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$ ./699419.pl
120 x 50

$ ./699419.pl </dev/null     # doesn't work
 x
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