Don't confuse the icon shown when a window is minimized, with an icon meant for informational display. If you read perldoc Tk::messageBox, it says it needs a Tk bitmap. Perldoc Tk::Bitmap specifies

   -data => string
     Specifies the contents of the source bitmap as a string.  The
     string must adhere to X11 bitmap format (e.g., as generated by th
+e     bitmap program).  If both the -data and -file options are
     specified,the -data option takes precedence.

    -file => name
     name gives the name of a file whose contents define the source bi
+t     map.  The file must adhere to X11 bitmap format (e.g., as gener
+ated
     by the bitmap program).
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