use strict;
my $str = '\\ref{eq:0And1Qubit} hjhg
\\ref{eq:EulerForm}the state is measured, as it can be shown $|\\rho e^{i\\theta}|^2=\\rho^2$ ($i$ is always the imaginary unit in this report). In other words, it is not possible to measure the global phase. The description of $\\ket{q}$ can then be confined to $\\alpha,\\beta,\\phi\\in\\mathbb{R}$, where $\\phi$ is the relative phase $\\phi=\\theta_\\beta-\\theta_\\alpha$.';
my $regex = qr/\\(ref){eq:(.*?)}/mp;
my $subst = '\\\\eqref{eq:$2}';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html