Error in perl matching

We got this error by running:

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my $str = "reb,cheng";

$str = ~ s/,//;

print $str,"n";

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seems everything is okay,but why?

Here is the error message:

C:Rebeccascriptperl>perl status_v0.2_clear.pl

Use of uninitialized value $_ in substitution (s///) at status_v0.2_clear.pl line 88,<DATA> line 751.

18446744073709551615

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Notice there is a space between “=” and “~”,the correct way should be NO SPACE between them:

my $str = "reb,"n";;

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then we got:

C:Rebeccascriptperl>perl status_v0.2_clear.pl

rebcheng

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It is executed successfully after removing the space!!