What you do in your code is getting names of files froma directory and then changing the in-memory strings, which has no effect on the actual file. Try this (untested):

my $dir = '.';

require File::Spec;

opendir my $dh,$dir or die "$dir - $!";

while(defined(my $f = readdir($dh))){
    local $_ = File::Spec->catpath(undef,$dir,$f);

    next unless -f;

    $f =~ tr/ \t/_/s;

    my $np = File::Spec->catpath(undef,$dir,$f);

    rename $_ => $np or warn "rename '$_' to '$np': $!";
}

closedir $dh;
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