You're localising $^I and @ARGV, localise $SIG{__WARN__} also:

sub inplace{
    my @files = @_;
   
    local($^I, @ARGV)=(".bak", $file); # Now i can use <>
    local $SIG{__WARN__} = sub{ ## Handle open errors here };
    while(<>){
        local $SIG{__WARN__}; # disable it for loop body
        # do stuff on $_
        print;
    }
}
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