Some suggestions:

• Indent your code properly. See perlstyle.
• Give your variables meaningful names. Extremely short variable names are "false economy".
• Perl understands real boolean values; use them. This lets you say if ( $sto ) rather than if ( $sto = 's' ).
• Learn to use arrays and hashes.
• Learn to use loops and subroutines for code re-use, rather than copy-and-paste.
• Don't set the width of the buttons. Or if you must, set them to something that allows the strings to be visible. A width of 1 makes a lot of your buttons unreadable. (Also, since width and height are numeric parameters, don't quote them.)

As an example of looping for code re-use, I rewrote part of your code into the following:

my %button_label =
(
    Sum    => '+',
    Subtra => '-',
    Multi  => 'x',
    Divi   => '/',
    Rs     => 'R/S',
);

my %placement =
(
    Enter  => [ 197,  155 ],
    Sum    => [ 353,  170 ],
    Subtra => [ 353,  140 ],
    Multi  => [ 353,  110 ],
    Divi   => [ 353,   80 ],
    CLx    => [ 150,  140 ],
    XsY    => [ 117,  140 ],
    Rs     => [  83,  140 ],
    SST    => [  50,  140 ],
    RbS    => [  17,  140 ],
    RCL    => [ 150,  170 ],
    STO    => [ 117,  170 ],
    OFF    => [  17,  170 ],
);


for (qw(
    Enter Sum Subtra Multi Divi
    CLx XsY Rs SST RbS RCL STO OFF
))
{
    my $w = $mw->Button( -text => $button_label{$_} || $_, )->pack;
    $w->place( -x => $placement{$_}[0], -y => $placement{$_}[1] );
}
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