You can just set the TZ environment variable to the timezone you want, and use localtime:

{
    local $ENV{TZ} = "EST+05";
    print "In New York, it is ".localtime()."\n";
}

{
    local $ENV{TZ} = "IST-05:30";
    print "In Mumbai, it is ".localtime()."\n"
}
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The last time I ran it, this printed

In New York, it is Mon Dec 19 14:25:01 2005
In Mumbai, it is Tue Dec 20 00:55:01 2005
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I think this will work on Windows as well as Unix, but I don't have a Windows box to test it on.