$b='This_is_Sample_Text';

if ($b=~/^(.+)_[^_]+$/)
{
    print "\n$1"; ##  This_is_Sample
}
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. = Match any character (except newline)

+ = Match 1 or more times

.+_ = Match upto last underscore(_)

[^_]+$ = Notmatch underscore(_) upto end of the string