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;