$ perl -wE 'my $str = "JA_PH"; $str =~ s/_/\s/; say $str'
Unrecognized escape \s passed through at -e line 1.
JAsPH
$ perl -wE 'my $str = "JA_PH"; $str =~ s/_/ /; say $str'
JA PH

##</code><code>##

$ perl -wE 'my $str = "Some text with  whitespace"; $str =~ s/\s /_/g; say $str'
Some text with_whitespace
$ perl -wE 'my $str = "Some text with  whitespace"; $str =~ s/\s/_/g; say $str'
Some_text_with__whitespace
$ perl -wE 'my $str = "Some text with  whitespace"; $str =~ s/\s+/_/g; say $str'
Some_text_with_whitespace