non of idea has worked

I demonstrated that my suggestion does work, so perhaps you could demonstrate with code how it doesn't do what you want? How do I post a question effectively?

here is a working code, anyone can try

my $req = HTTP::Request->new(GET => 'https://www.google.com/finance/co
+nverter?a=3&from=USD&to=EUR');
$req->content_type('application/json');
my $re = $ua->request($req);
  $re->content =~ m|<span class=bld>(.+?)</span>|i and $span = $1;
  $ct = substr($1, 0, -6);
  print $ct;
  prints 2.77 according to the currency  usd to euro
[download]

but when you try to put amount from string $m = "3";

  https://www.google.com/finance/converter?a=$m&from=USD&to=EUR
[download]

doesn;t work correctly

People have tried to explain but we are not getting thru to you, look at this

use strict; use warnings; 
use LWP; 
my $ua = new LWP::UserAgent();

{
# In this case you have 2 literal strings enclosed in single quotes 
#  with the variable $m concatenated in the middle. 
# I like that way best because you are assured text is text and variab
+les are variables. 
#
my $m=3; 
my $uri = 'https://www.google.com/finance/converter?a='.$m.'&from=USD&
+to=EUR'; 
my $req = HTTP::Request->new(GET => $uri);
$req->content_type('application/json');
my $re = $ua->request($req);
$re->content =~ m|<span class=bld>(.+?)</span>|i;#  and $span = $1;
  my $ct = substr($1, 0, -6);
  print 'ct:'.$ct."\n";
print $uri."\n"; 
}


{
# Here you have one string, 
#   but because it is in double quotes perl symbols/expressions get in
+terpolated inside. 
# Some think it is cleaner, 
#  but things you dont expect can change 
#    because perl sees them as something to replace. 
#
my $m=3; 
my $uri="https://www.google.com/finance/converter?a=$m&from=USD&to=EUR
+"; 
print $uri."\n"; 
}

{
# a third way, as suggested by [haukex]
# In this case many problems associated with special characters in the
+ uri are avoided.
use URI;
my $m=3; 
my $uri =URI->new('https://www.google.com/finance/converter');
$uri->query_form(
    a     => $m,
    from  => 'USD',
    to    => 'EUR',
    );
print $uri."\n"; 
}
[download]

Notice how all 3 printed uris are the same and what you want to see

D:\goodies\pdhuck\down1\perl\monks>perl uri.pl
ct:2.78
https://www.google.com/finance/converter?a=3&from=USD&to=EUR
https://www.google.com/finance/converter?a=3&from=USD&to=EUR
https://www.google.com/finance/converter?a=3&from=USD&to=EUR
[download]

Do you get it yet? I only ran the first method but sine all 3 printed uris are the same any of the 3 methods would work the same.