On using $_ in subroutines

Is it not possible to use $_ in subroutines?

For example, my temptation was to do this:

**************************************
sub isDate {

    $_ = shift;
    if (m!\d{2}/\d{2}/\d{2}!) { return 1; }
    else { return 0; }


} 
**************************************

... but by modifying $_ I was clobbering $_ elsewhere in the larger program!
Oddly, perl won't let me do "my ($_) = shift;", so I'm stuck having to use
another variable.

Why can't we do that?  Is using $_ in subroutines discouraged??

TIA.

- Bryan



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