The (\w) will match and capture a digit which doesn't exist in the hash so will be lost in the substitution. Without a look-behind you will also consume and lose the digit before. Better I think to use a character class along with the look-behind.

$ perl -E '
$val = q{2.73M};
%exp = ( M => 6 );
$val =~ s/\d(\w)/exists $exp{ $1 } ? q{e} . $exp{ $1 } : q{} /e;
say $val;'
2.M
$ perl -E '
$val = q{2.73M};
%exp = ( M => 6 );
$val =~ s/(?<=\d)(\w)/exists $exp{ $1 } ? q{e} . $exp{ $1 } : q{} /e;
say $val;'
2.7M
$ perl -E '
$val = q{2.73M};
%exp = ( M => 6 );
$val =~ s/(?<=\d)([TGMkmunp])/exists $exp{ $1 } ? q{e} . $exp{ $1 } : 
+q{} /e;
say $val;'
2.73e6
~ $
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I hope this is of interest.